1. Technical Field of the Invention
The present invention relates to a wireless communications system and, in particular, to reducing mobile station power consumption in connection with paging and mobile station channel measurement operations.
2. Description of Related Art
It is well known that mobile stations (telephones) in wireless communications systems consume large amounts of power while operating in a talk mode. Consumption of power at a significant, albeit reduced, rate continues when the mobile station operates in a stand-by (i.e., idle) mode awaiting receipt or origination of a telephone call. For currently available portable, battery powered mobile stations, the on-board battery typically has a working lifetime of approximately eight hours in the stand-by mode and two hours in the talk mode. After such time expires, the battery must be recharged or replaced in order for the mobile station to continue to provide communications service.
Many methods and apparatus have been proposed to reduce power consumption in mobile stations and thus extend battery life between charges or replacement. A functional characteristic shared in common by many of these methods and apparatus is controlling mobile station operation by denying or reducing the supply of power to certain electronic components of the mobile station such as the transceiver, display or processor in order to reduce drain on the battery and extend useful battery life. This is commonly referred to as "sleep mode" operation. In order to obtain the maximum power conservation benefit from sleep mode operation, it is important to maximize the amount of time the supply of power is denied or reduced. Efficient mobile station operation, however, depends on mobile station monitoring of control and command signals transmitted from the serving wireless communications system as well as making downlink signal strength measurements. These monitoring and measurement functions require the mobile station to be adequately powered to enable transceiver and processor operation. Accordingly, a balance must be struck between power conservation (sleep mode) operation and power consumption operation. Given the availability of sleep mode functionality, a need exists for a method and apparatus by which the time spent in sleep mode can be maximized without adversely affecting the ability of the mobile station to be sufficiently operational to carry out any required monitoring and measurement functions.
Reference is now made to FIG. 1 wherein there is shown the structure of a hyperframe 10 for the forward (i.e., downlink) digital control channel (F-DCCH) as specified by the TIA/EIA Interim Standard IS-136 air interface. The hyperframe 10 is composed of two superframes 12. A first one these superframes 12(1) is commonly referred to as the "primary" superframe. A second one these superframes 12(2) is commonly referred to as the "secondary" superframe. Each superframe 12 is composed of a number of logical channels. A first one of these logical channels is a broadcast control channel (BCCH) commonly referred to as the fast broadcast control channel (F-BCCH) 14. This logical channel is used to broadcast control channel structure parameters and parameters that are essential for accessing the system. A next one of these logical channels is a broadcast control channel commonly referred to as the extended broadcast control channel (E-BCCH) 16. This logical channel carries broadcast information to mobile stations that is less time critical than the F-DCCH information. Another one of these logical channels is a broadcast control channel commonly referred to as the short message service (SMS) broadcast control channel (S-BCCH) 18. This logical channel is used to broadcast short message service messages to mobile stations. Yet another one of these logical channels is commonly referred to as the SMS point-to-point, paging and access response channel (SPACH) 20. This logical channel is used to broadcast information to specific mobile stations regarding SMS messages, paging messages, and to provide an access response channel. Another defined logical channel (RES) 22 is reserved at this point in time for a future but not yet specified use.
The SMS point-to-point, paging and access response channel 20 is divided into three logical sub-channels. A first one of these sub-channels is the paging channel (PCH) 24 that is dedicated to delivering pages and orders. A next one of these sub-channels is the access response channel (ARCH) 26 that is used to convey assignments to another communications resource or other responses to a mobile station access attempt. Another one of these sub-channels is the SMS channel (SMSCH) 28 that is used to deliver short message service messages to a specific mobile station.
One superframe 12 includes thirty-two slots 30 for use by the broadcast control channels (BCCH) 14, 16 and 18, the reserved channel (RES) 22, and the SMS point-to-point, paging and access response channel (SPACH) 20. Of these thirty-two available slots 30, when full rate operation is specified, between one and twenty-eight slots are made available to the SMS point-to-point, paging and access response channel 20 for use by the paging channels (PCH) 24, access response channels (ARCH) 26 and the SMS channels (SMSCH) 28. Any of the slots 30 used for paging channels 22 in the primary superframe 12(1) are repeated for use in the secondary superframe 12(2). The other SMS point-to-point, paging and access response channel 20 related information, such as with respect to the access response channel 26 and the SMS channel 28, may be different from one superframe 12 to a next superframe.
Each mobile station is assigned a particular one of the slots 30 in the SMS point-to-point, paging and access response channel 20 as its paging channel (PCH) 24. A mobile station camped on to the forward digital control channel reads its assigned paging channel 24 to determine whether a message has been broadcast addressed to its mobile station identification (MSID). If there is no such message detected, the mobile station further reads a page continuation (PCON) bit carried in the slot 30 for its assigned paging channel 24. If the page continuation bit is clear (PCON=0), the mobile station enters sleep mode until the next occurrence of its assigned paging channel 24 on to the forward digital control channel (not shown). If, on the other hand, the page continuation bit is set (PCON=1), a page displacement functionality is activated and the mobile station responds by delaying entry into sleep mode in order to read additional slots 30 in the SMS point-to-point, paging and access response channel 20. The number of slots 30 which are then read is specified by a paging channel displacement parameter (PCH.sub.-- DISPLACEMENT) sent over the forward digital control channel.
When full rate operation is specified, the mobile station reads every other slot 30 in the SMS point-to-point, paging and access response channel 20 following its assigned paging channel 24 slot until the number of slots read equals the number specified in the paging channel displacement parameter or it receives a paging message addressed to its mobile station identification. In the situation where the mobile station has read the last available slot 30 in the SMS point-to-point, paging and access response channel 20 of the primary superframe 12(1), and the paging displacement parameter specifies that additional slots be read, the mobile station starts again reading with the second slot in the SMS point-to-point, paging and access response channel of the next primary superframe. In the situation where the mobile station has read the next-to-last available slot 30 in the SMS point-to-point, paging and access response channel 20 of the primary superframe 12(1), and the paging displacement parameter specifies that additional slots be read, the mobile station starts again reading with the first slot in the SMS point-to-point, paging and access response channel of the next primary superframe.
An example of paging displacement operation is shown in FIG. 2. In this example of full rate operation for the forward digital control channel, the SMS point-to-point, paging and access response channel 20 is assigned six slots 30(1)-30(6). Assume that a given mobile station is assigned a paging channel 24 that occurs in slot 30(1) of the first illustrated primary superframe 12(1). The mobile station wakes up and reads this paging channel 24 (as indicated by "X") to determine whether a message has been broadcast addressed to its mobile station identification (MSID). If there is no such message detected, the mobile station further reads a page continuation (PCON) bit. If the page continuation bit is clear (PCON=0), the mobile station enters sleep mode until the next occurrence (not shown) of its assigned paging channel 24 on the forward digital control channel. If, on the other hand, the page continuation bit is set (PCON=1), the mobile station reads every other slot (30(3), 30(5)) in the SMS point-to-point, paging and access response channel 20 following the slot 30(1) for its assigned paging channel 24 (as indicated by "Y") until the number of slots read equals the number specified in the paging channel displacement parameter (PCH.sub.-- DISPLACEMENT=4 in this example) or it receives a paging message addressed to its mobile station identification. When the mobile station reads the next-to-last available slot 30(5) in the SMS point-to-point, paging and access response channel 20 of the first illustrated primary superframe 12(1), and the paging displacement parameter specifies that additional slots be read, the mobile station starts again reading with the first slot 30(1) in the SMS point-to-point, paging and access response channel of the second illustrated (i.e., the next following) primary superframe.
Not only must a mobile station wake up to read its assigned paging channel, it must also wake up periodically to make signal strength measurements. Although these measurements are not required to be performed at any particular place in the superframe, they are typically performed immediately before or after the paging data. These measurements are made on the control channels of cells in the wireless communications system which neighbor the cell currently serving the mobile station. The signal strength measurements are then processed by the mobile station for the purpose of making server selection determinations.
Paging displacement provides a mechanism by which the wireless communications system can force mobile stations to postpone entry into sleep mode and make additional reads on the forward digital control channel for the purpose of finding pages. Postponing entry into sleep mode and requiring transceiver and processor operation to make additional control channel reads places additional demands on the stored energy resources of the mobile station battery. Even the reading of a few extra slots can significantly shorten battery life. The additional periodic requirement to make signal strength measurements further serves as a drain on the limited energy resources of the battery. A method and apparatus are needed to maximize the time spent in sleep mode so as to conserve battery resources while simultaneously facilitating mobile station operation to make requisite signal strength measurements and support the paging channel displacement functionality.